NEWS | Aug. 29, 2011

Mixing Hot Spot in the Gulf of Mexico

By Donna McKinney

To improve oceanographers' understanding of the structure and dynamics of small-scale to sub-mesoscale mixing processes over rough, bathymetric features on the continental shelf, members of the Naval Research Laboratory's Oceanography Division are leading an extensive measurements program focused on the East Flower Garden Bank, one of three areas that make up the Flower Garden Banks National Marine Sanctuary (FGBNMS) in the northwestern Gulf of Mexico.

According to NOAA, who oversees the FGBNMS, fishermen nicknamed the area the Texas Flower Gardens because of the brightly colored sponges, plants, and other marine life they saw on the reefs. Banks refers to the salt dome formations upon which the reefs sit. Manta rays, whale sharks, large coral heads, hundreds of species of fish and invertebrates live in this wild but beautiful environment. The East Flower Garden Bank covers approximately 25 of the sanctuary's total 56 square miles.

Flower Gardens Are a Hot Spot
The NRL researchers' main objective of this expedition was to examine the importance of the topographic induced processes on shelf-edge circulation on timescales ranging from seasonal to minutes and to gather data unique to the East Flower Gardens Bank.

The NRL project--Mixing Over Rough Topography (MORT)--is executed in conjunction with a closely related project of the Bureau of Ocean Energy Management Regulation and Enforcement (BOEMRE). Both projects are of high interest in the Gulf of Mexico following the recent Deepwater Horizon oil spill.

Bathymetric features, such as the coral reefs in the East Flower Garden Bank, are hot spots in mixing in coastal zones, said Dr. Hemantha Wijesekera, an oceanographer in NRL's Physical Oceanographic Processes Branch. Their interaction with mean flows leads to enhanced lateral and vertical mixing in the water column. Impacts of these bumps affect physical, bio-optical, acoustic, and electromagnetic properties in the coastal ocean.

The mixing processes are not limited to a particular geographical location; they can apply anywhere in the world along the continental shelves. For the next stage of numerical ocean model development, small-scale physical processes must first be understood to truly advance to sub-kilometer scales.

Six Cruises, Two Ships, One Year
Cooperating on fieldwork benefits all the parties involved, but the work requires close coordination among NRL, BOEMRE, and NOAA's FGBNMS to determine sharing and contribution of resources, timing of the field work, and specific study site selection.

Scientists on the first Pelican cruise in December 2010 deployed moorings to measure the currents and density structure around the East Flower Garden Bank. The moorings are scheduled to take measurements until December 2011.

On the second Pelican cruise (May/June 2011), the moorings were recovered, serviced, and redeployed. Scientists also deployed and recovered high-frequency current Barny moorings and temperature/conductivity string moorings, released fluorescein dye to observe mixing and surveyed the area using ScanFish (a towed water column 3d profiler) and REMUS (an autonomous underwater vehicle equipped with a variety of sensors).

The third Pelican cruise in December 2011 will recover the moorings.

R/V Manta joined Pelican for a week in June, during which time the researchers on board focused efforts on mixing measurements. The final two Manta cruises are planned in August and December 2011.

Over the entire experiment, NRL deployed a total of 15 Barny and six string moorings.

A Barny mooring resembles a 2-m diameter barnacle resting on the sea floor, which makes it highly resistant to trawling and other fishing gear. An outer ring of reinforced cement provides additional impact resistance and ballast.

Contained in the Barny at its top in a small buoy is an acoustic Doppler current profiler (ADCP). The Barny also contains either a wave/tide gauge or a high-frequency pressure p-pod sensor, temperature, and conductivity sensors in the body. Once equipped, the mooring is lowered to the seafloor and released.

At the end of the deployment, the mooring is recovered via an acoustic release of the small buoy, which carries a recovery line and the ADCP to the sea surface. The main body of the mooring is hauled up using the recovery line.

The string moorings consisted of 8 to10 MicroCats, temperature and conductivity recorders. (Some will also measure pressure.) The string moorings are anchored on the bottom and use subsurface floats to pull the mooring taut.

In addition, during the first Pelican cruise, researchers used NRL's Vertical Microstructure Profiler (VMP) to collect one section of microstructure profiles to study turbulence levels. The VMP is equipped with state-of-art microstructure velocity shear probes, high-resolution temperature and conductivity sensors, and external high-accuracy CTD sensors.

The Manta collected more than 500 VMP profiles, along with optics, ADCP and acoustic backscatter data.

To capture mixing data, Fluorescein, a brightly colored fluorescent dye tracer, was released at two different depths. Researchers then deployed the REMUS and towed ScanFish from the ship in a profiling mode to measure temperature and salinity, and inherent optical properties.

The two vehicles were used to conduct a profiling microstructure survey to capture the downstream distribution of the dye. The dye patch will reveal fine-scale to mesoscale features and can be used to quantify horizontal (isopycnal) and vertical (diapycnal) diffusion.

The data researchers gather from this experiment will be used to quantify important small-scale to sub-mesoscale processes over rough bathymetric features. The analysis of data will improve subgrid scale parameterizations of buoyancy and momentum transports, and bio-optical property distributions in coastal ocean models.

Researchers and Collaborators
The Pelican science party included William Teague, Hemantha Wijesekera, Mark Hulbert, Andrew Quaid, Justin Brodersen and Steve Sova (Qinetiq) from NRL's Physical Oceanographic Processes Branch; and Ian Robbins and Jason Felton from California Polytechnic State University.

The Manta science party included Ewa Jarosz, Diane Bennett Fribance and Andrew Quaid (transferred from Pelican) NRL's Physical Oceanographic Processes Branch; and Alan Weidemann and Wesley Goode from NRL's Ocean Optical Processes and Remote Sensing Branch.

Other collaborators include:
  • J. Moum from Oregon State University, who through existing ONR support provided pressure sensors (P-PODs) to investigate the pressure fields associated with topographically generated internal waves.
  • M. Moline (Cal Poly), who was funded by ONR to deploy a REMUS autonomous underwater vehicle to focus on the detection of dye patches.
  • Michael Gregg and Tim McGinnis from the University of Washington Applied Physics Laboratory, who were funded by ONR to tow SWIMS3 (turbulence) on a separate vessel during a fall experiment.
  • Jason Jolliff from NRL's Coupled Bio-Optical Physical Processes Branch, who was funded by NASA for high-resolution physical-optical mapping and modeling.
  • Derek Burrage and Joel Wesson, NRL's Physical Oceanographic Processes Branch, who performed over flights during dye dispersals as part of NRL's Surface Roughness Impacts on Microwave Sea Surface Salinity Measurements (SRIMS) project.